Azimuthally-Selective Downhole Nuclear Magnetic Resonance (NMR) Tool

ABSTRACT

In some aspects, a downhole nuclear magnetic resonance (NMR) tool includes a magnet assembly and an antenna assembly. The NMR tool can operate in a wellbore in a subterranean region to obtain NMR data from the subterranean region. The magnet assembly produces a magnetic field in a volume about the wellbore. The antenna assembly produces excitation in the volume and acquires an azimuthally-selective response from the volume based on the excitation. The antenna assembly can include a transversal-dipole antenna and a monopole antenna.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/872,362, filed on Aug. 30, 2013, entitled “Obtaining NuclearMagnetic Resonance (NMR) Data from a Subterranean Region.” The priorityapplication is hereby incorporated by reference in this application.

BACKGROUND

This specification relates to azimuthally-selective downhole nuclearmagnetic resonance (NMR) tools, for example, for obtaining NMR data froma subterranean region.

In the field of logging (e.g. wireline logging, logging while drilling(LWD) and measurement while drilling (MWD)), nuclear magnetic resonance(NMR) tools have been used to explore the subsurface based on themagnetic interactions with subsurface material. Some downhole NMR toolsinclude a magnet assembly that produces a static magnetic field, and acoil assembly that generates radio frequency (RF) control signals anddetects magnetic resonance phenomena in the subsurface material.Properties of the subsurface material can be identified from thedetected phenomena.

DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram of an example well system.

FIG. 1B is a diagram of an example well system that includes an NMR toolin a wireline logging environment.

FIG. 1C is a diagram of an example well system that includes an NMR toolin a logging while drilling (LWD) environment.

FIG. 2A is a diagram of an example downhole tool for obtaining NMR datafrom a subterranean region.

FIG. 2B is a diagram of another example downhole tool for obtaining NMRdata from a subterranean region.

FIG. 3A is a plot showing azimuthal selectivity for an example downholetool.

FIG. 3B is a diagram of another example downhole tool for obtaining NMRdata from a subterranean region.

FIG. 4A is a flowchart showing an example technique for obtaining NMRdata from a subterranean region.

FIG. 4B is a flowchart showing another example technique for obtainingNMR data from a subterranean region.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

In some implementations, an NMR instrument can offer practical solutionsfor obtaining NMR data from the subsurface. In some instances, theinstrument can provide a higher signal-to-noise ratio (SNR) (e.g., for agiven DC power budget), motional immunity, azimuthal selectivity ofmeasurements, or a combination of these or other advantages. In somecases, the instrument can be robust against environmental factors, andprovide accurate or precise information for analysis of the subsurface.

Some example configurations for a downhole NMR instrument include asubstantially two-dimensional (2D) transversal dipole arrangement forboth the magnet assembly and the antenna assembly. The magnetic fieldsgenerated by the magnet and antennas can have axial homogeneity (i.e.,homogeneity along the long axis of the NMR instrument) that is suitablefor use during axial motion. In some cases, broader band excitation(saturation of nuclear magnetization) can be used, for example, toachieve axial symmetry (roundness) with this type of instrument. In someimplementations, a downhole NMR tool is configured to generateaxially-symmetric magnetic fields, with a magnet assembly generating aradial magnetic field and an antenna assembly generating a longitudinalRF magnetic field (also having a longitudinal sensitivity direction).

In some instances, an NMR instrument can produce a longitudinal staticmagnetic field in the volume of interest. In some examples, theinstrument includes multiple transversal-dipole antennas (e.g., twoidentical transversal-dipole antennas) that produce circular polarizedexcitation and provide quadrature coil detection. An arrangement ofmultiple orthogonal antennas can be used, for example, with alongitudinal-dipole magnet that generates an axial static magnetic fieldin the volume of interest. In some examples, the instrument includes amultiple-volume arrangement that makes use of different regions of themagnet assembly to acquire the NMR signal. In some examples, a region ofinvestigation has a shape that is suitable for measurements whiletripping the drill string (i.e., transiting the drill string in thewellbore). Some example implementations include a combination of atransversal-dipole antenna axially-symmetrical response and a monopoleantenna axially-symmetrical response, which can enableazimuthally-resolved unidirectional NMR measurements in some instances.

FIG. 1A is a diagram of an example well system 100 a. The example wellsystem 100 a includes an NMR logging system 108 and a subterraneanregion 120 beneath the ground surface 106. A well system can includeadditional or different features that are not shown in FIG. 1A. Forexample, the well system 100 a may include additional drilling systemcomponents, wireline logging system components, etc.

The subterranean region 120 can include all or part of one or moresubterranean formations or zones. The example subterranean region 120shown in FIG. 1A includes multiple subsurface layers 122 and a wellbore104 penetrated through the subsurface layers 122. The subsurface layers122 can include sedimentary layers, rock layers, sand layers, orcombinations of these and other types of subsurface layers. One or moreof the subsurface layers can contain fluids, such as brine, oil, gas,etc. Although the example wellbore 104 shown in FIG. 1A is a verticalwellbore, the NMR logging system 108 can be implemented in otherwellbore orientations. For example, the NMR logging system 108 may beadapted for horizontal wellbores, slanted wellbores, curved wellbores,vertical wellbores, or combinations of these.

The example NMR logging system 108 includes a logging tool 102, surfaceequipment 112, and a computing subsystem 110. In the example shown inFIG. 1A, the logging tool 102 is a downhole logging tool that operateswhile disposed in the wellbore 104. The example surface equipment 112shown in FIG. 1A operates at or above the surface 106, for example, nearthe well head 105, to control the logging tool 102 and possibly otherdownhole equipment or other components of the well system 100. Theexample computing subsystem 110 can receive and analyze logging datafrom the logging tool 102. An NMR logging system can include additionalor different features, and the features of an NMR logging system can bearranged and operated as represented in FIG. 1A or in another manner.

In some instances, all or part of the computing subsystem 110 can beimplemented as a component of, or can be integrated with one or morecomponents of, the surface equipment 112, the logging tool 102 or both.In some cases, the computing subsystem 110 can be implemented as one ormore computing structures separate from the surface equipment 112 andthe logging tool 102.

In some implementations, the computing subsystem 110 is embedded in thelogging tool 102, and the computing subsystem 110 and the logging tool102 can operate concurrently while disposed in the wellbore 104. Forexample, although the computing subsystem 110 is shown above the surface106 in the example shown in FIG. 1A, all or part of the computingsubsystem 110 may reside below the surface 106, for example, at or nearthe location of the logging tool 102.

The well system 100 a can include communication or telemetry equipmentthat allows communication among the computing subsystem 110, the loggingtool 102, and other components of the NMR logging system 108. Forexample, the components of the NMR logging system 108 can each includeone or more transceivers or similar apparatus for wired or wireless datacommunication among the various components. For example, the NMR loggingsystem 108 can include systems and apparatus for optical telemetry,wireline telemetry, wired pipe telemetry, mud pulse telemetry, acoustictelemetry, electromagnetic telemetry, or a combination of these andother types of telemetry. In some cases, the logging tool 102 receivescommands, status signals, or other types of information from thecomputing subsystem 110 or another source. In some cases, the computingsubsystem 110 receives logging data, status signals, or other types ofinformation from the logging tool 102 or another source.

NMR logging operations can be performed in connection with various typesof downhole operations at various stages in the lifetime of a wellsystem. Structural attributes and components of the surface equipment112 and logging tool 102 can be adapted for various types of NMR loggingoperations. For example, NMR logging may be performed during drillingoperations, during wireline logging operations, or in other contexts. Assuch, the surface equipment 112 and the logging tool 102 may include, ormay operate in connection with drilling equipment, wireline loggingequipment, or other equipment for other types of operations.

In some implementations, the logging tool 102 includes a magnet assemblythat includes a central magnet and two end piece magnets. Examples areshown in FIGS. 2A, 2B, and 3B. The end piece magnets can be spaced apartfrom the axial ends of the central magnet. The end pieces together withthe central magnets can define four magnetic poles, which may bearranged to enhance the static magnetic field in a volume of interest.In some cases, the central magnet defines a first magnetic fieldorientation, and the end piece magnets define a second magnetic fieldorientation that is orthogonal to the first magnetic field orientation.The logging tool 102 can also include multiple orthogonaltransversal-dipole antennas. The orthogonal transversal-dipole antennascan produce circular polarized excitation in a subterranean volume andacquire a response from the volume by quadrature coil detection.

In some implementations, the logging tool 102 includes a magnet assemblythat produces a magnetic field in multiple distinct sub-volumes in thesubterranean region 120. An example is shown in FIG. 2B. A firstsub-volume can be an elongate cylindrical-shell region that extends inthe longitudinal direction (parallel to the wellbore axis), and themagnetic field in the first sub-volume can be substantially uniformlyoriented along the longitudinal direction. Second and third sub-volumescan be spaced apart from the axial ends of the first sub-volume, and thestatic magnetic field in the second and third sub-volumes can have aradial orientation (perpendicular to the longitudinal direction). Thesecond and third sub-volumes can be located at a different distance fromthe center of the tool string than the first volume. In some instances,the locations of the second and third sub-volumes allow the logging toolto collect information for mud filtrate invasion profiling. The loggingtool 102 can also include multiple antenna assemblies at respectivelocations along the longitudinal axis. Each of the antenna assembliescan detect an NMR response from a respective one of the distinctsub-volumes.

In some implementations, the logging tool 102 includes a magnet assemblyand a transversal-dipole and monopole antenna assembly. An example isshown in FIG. 3B. The transversal-dipole and monopole antenna assemblycan obtain a unidirectional azimuthally-selective NMR response from asubterranean volume about the magnet assembly. The transversal-dipoleand monopole antenna assembly can include orthogonal transversal-dipoleantennas and a monopole antenna.

In some examples, NMR logging operations are performed during wirelinelogging operations. FIG. 1B shows an example well system 100 b thatincludes the logging tool 102 in a wireline logging environment. In someexample wireline logging operations, the surface equipment 112 includesa platform above the surface 106 equipped with a derrick 132 thatsupports a wireline cable 134 that extends into the wellbore 104.Wireline logging operations can be performed, for example, after a drillstring is removed from the wellbore 104, to allow the wireline loggingtool 102 to be lowered by wireline or logging cable into the wellbore104.

In some examples, NMR logging operations are performed during drillingoperations. FIG. 1C shows an example well system 100 c that includes thelogging tool 102 in a logging while drilling (LWD) environment. Drillingis commonly carried out using a string of drill pipes connected togetherto form a drill string 140 that is lowered through a rotary table intothe wellbore 104. In some cases, a drilling rig 142 at the surface 106supports the drill string 140, as the drill string 140 is operated todrill a wellbore penetrating the subterranean region 120. The drillstring 140 may include, for example, a kelly, drill pipe, a bottomholeassembly, and other components. The bottomhole assembly on the drillstring may include drill collars, drill bits, the logging tool 102, andother components. The logging tools may include measuring while drilling(MWD) tools, LWD tools, and others.

In some implementations, the logging tool 102 includes an NMR tool forobtaining NMR measurements from the subterranean region 120. As shown,for example, in FIG. 1B, the logging tool 102 can be suspended in thewellbore 104 by a coiled tubing, wireline cable, or another structurethat connects the tool to a surface control unit or other components ofthe surface equipment 112. In some example implementations, the loggingtool 102 is lowered to the bottom of a region of interest andsubsequently pulled upward (e.g., at a substantially constant speed)through the region of interest. As shown, for example, in FIG. 1C, thelogging tool 102 can be deployed in the wellbore 104 on jointed drillpipe, hard wired drill pipe, or other deployment hardware. In someexample implementations, the logging tool 102 collects data duringdrilling operations as it moves downward through the region of interest.In some example implementations, the logging tool 102 collects datawhile the drill string 140 is moving, for example, while it is beingtripped in or tripped out of the wellbore 104.

In some implementations, the logging tool 102 collects data at discretelogging points in the wellbore 104. For example, the logging tool 102can move upward or downward incrementally to each logging point at aseries of depths in the wellbore 104. At each logging point, instrumentsin the logging tool 102 perform measurements on the subterranean region120. The measurement data can be communicated to the computing subsystem110 for storage, processing, and analysis. Such data may be gathered andanalyzed during drilling operations (e.g., during logging while drilling(LWD) operations), during wireline logging operations, or during othertypes of activities.

The computing subsystem 110 can receive and analyze the measurement datafrom the logging tool 102 to detect properties of various subsurfacelayers 122. For example, the computing subsystem 110 can identify thedensity, viscosity, porosity, material content, or other properties ofthe subsurface layers 122 based on the NMR measurements acquired by thelogging tool 102 in the wellbore 104.

In some implementations, the logging tool 102 obtains NMR signals bypolarizing nuclear spins in the subterranean region 120 and pulsing thenuclei with a radio frequency (RF) magnetic field. Various pulsesequences (i.e., series of radio frequency pulses, delays, and otheroperations) can be used to obtain NMR signals, including the CarrPurcell Meiboom Gill (CPMG) sequence (in which the spins are firsttipped using a tipping pulse followed by a series of refocusing pulses),the Optimized Refocusing Pulse Sequence (ORPS) in which the refocusingpulses are less than 180°, a saturation recovery pulse sequence, andother pulse sequences.

The acquired spin-echo signals (or other NMR data) may be processed(e.g., inverted, transformed, etc.) to a relaxation-time distribution(e.g., a distribution of transverse relaxation times T₂ or adistribution of longitudinal relaxation times T₁), or both. Therelaxation-time distribution can be used to determine various physicalproperties of the formation by solving one or more inverse problems. Insome cases, relaxation-time distributions are acquired for multiplelogging points and used to train a model of the subterranean region. Insome cases, relaxation-time distributions are acquired for multiplelogging points and used to predict properties of the subterraneanregion.

FIG. 2A is a diagram of an example NMR tool 200A. The example NMR tool200A includes a magnet assembly that generates a static magnetic fieldto produce polarization, and an antenna assembly that (a) generates aradio frequency (RF) magnetic field to generate excitation, and (b)acquires NMR signals. In the example shown in FIG. 2A, the magnetassembly that includes the end piece magnets 11A, 11B and a centralmagnet 12 generates the static magnetic field in the volume ofinvestigation 17. In the volume of investigation 17, the direction ofthe static magnetic field (shown as the solid black arrow 18) isparallel to the longitudinal axis of the wellbore. In some examples, amagnet configuration with double pole strength can be used to increasethe strength of the magnetic field (e.g., up to 100-150 Gauss or higherin some instances).

In the example shown in FIG. 2A, the antenna assembly 13 includes twomutually orthogonal transversal-dipole antennas 15, 16. In someinstances, the NMR tool 200A can be implemented with a singletransversal-dipole antenna. For example, one of the transversal-dipoleantennas 15, 16 may be omitted from the antenna assembly 13. The exampletransversal-dipole antennas 15, 16 shown in FIG. 2A are placed on anouter surface of a soft magnetic core 14, which is used for RF magneticflux concentration. The static magnetic field can be axially symmetric(or substantially axially symmetric), and therefore may not requirebroader band excitation associated with additional energy loss. Thevolume of investigation can be made axially long enough and thick enough(e.g., 20 cm long, and 0.5 cm thick in some environments) to provideimmunity or otherwise decrease sensitivity to axial motion, lateralmotion, or both. A longer sensitivity region can enable measurementwhile tripping the drill string. The sensitivity region can be shaped byshaping the magnets 11A, 11B, 12 and the soft magnetic material of thecore 14.

In some implementations, the antenna assembly 13 additionally oralternatively includes an integrated coil set that performs theoperations of the two transversal-dipole antennas 15, 16. For example,the integrated coil may be used (e.g., instead of the twotransversal-dipole antennas 15, 16) to produce circular polarization andperform quadrature coil detection. Examples of integrated coil sets thatcan be adapted to perform such operations include multi-coil or complexsingle-coil arrangements, such as, for example, birdcage coils commonlyused for high-field magnetic resonance imaging (MRI).

Compared to some example axially-symmetrical designs, the use of thelongitudinal-dipole magnet and the transversal-dipole antenna assemblyalso has an advantage of less eddy current losses in the formation anddrilling fluid (i.e., “mud”) in the wellbore due to a longer eddycurrent path than for some longitudinal-dipole antenna(s).

In some aspects, NMR measurements over multiple sub-volumes can increasethe data density and therefore SNR per unit time. Multiple volumemeasurements in a static magnetic field having a radial gradient can beachieved, for example, by acquiring NMR data on a second frequency whilewaiting for nuclear magnetization to recover (e.g., after a CPMG pulsetrain) on a first frequency. A number of different frequencies can beused to run a multi-frequency NMR acquisition involving a number ofexcitation volumes with a different depth of investigation. In additionto higher SNR, the multi-frequency measurements can also enableprofiling the fluid invasion in the wellbore, enabling a betterassessment of permeability of earth formations. Another way to conductmulti-volume measurements is to use different regions of the magnetassembly to acquire an NMR signal. NMR measurements of these differentregions can be run at the same time (e.g., simultaneously) or atdifferent times.

FIG. 2B is a diagram of another example NMR tool 200B. The example NMRtool 200B also includes a magnet assembly that generates a staticmagnetic field to produce polarization, and an antenna assembly that (a)generates a radio frequency (RF) magnetic field to generate excitation,and (b) acquires NMR signals. In the example shown in FIG. 2B, themagnet assembly produces a magnetic field having a dominant axialcomponent in the volume of investigation 21. The directions of the RFmagnetic field (produced by two transversal dipole antennas as in FIG.2A) and the static magnetic field in this region are shown at 22. In theexample shown in FIG. 2B, two distinct volumes of investigation 24A, 24Bare created near the magnet poles (beyond the axial ends of the centralmagnet) where the static magnetic field has a predominantly radialcomponent. The example NMR antennas shown at 23A and 23B can generate RFmagnetic fields in the volumes of investigation 24A and 24B near thelongitudinal-dipole antennas. The longitudinal direction of the RFmagnetic fields in the volumes of investigation 24A and 24B, and theradial direction of the static magnetic field in the volumes ofinvestigation 24A and 24B, are shown at 25A and 25B.

In some aspects, a combination of transversal-dipole and monopoleantennas can be used to enable unidirectional azimuthally-selectivemeasurements, without substantially reducing SNR in some cases. In someexamples, the NMR excitation can be substantially axially symmetrical(e.g., using either the transversal-dipole antenna or the monopoleantenna) while a combination of axially-symmetrical sensitivitytransversal-dipole antenna and the axially-symmetrical sensitivitymonopole antenna responses can enable azimuthally-resolved measurements.

FIGS. 3A and 3B illustrate aspects of an example azimuthally-selectiveNMR tool. FIG. 3A is a plot 300A showing an example of azimuthallyselected data from the example downhole tool 300B shown in FIG. 3B. Theexample NMR tool 300B includes a magnet assembly that generates a staticmagnetic field to produce polarization, and an antenna assembly that (a)generates a radio frequency (RF) magnetic field to generate excitation,and (b) acquires NMR signals. The antenna assembly 31 shown in FIG. 3Bincludes a monopole antenna and two orthogonal transversal-dipoleantennas 35 and 36. The example monopole antenna includes two coils 37Aand 37B connected in reverse polarity in order to generate asubstantially radial RF magnetic field in the volume of investigation34. Due to reciprocity, the same coil arrangement can have a radialsensitivity direction. The example RF magnetic fields BRF presented at32 and 33 can reflect the total sensitivity direction when the monopoleantenna response is combined with one of the transversal-dipole antennaresponses.

The example monopole antenna shown in FIG. 3B includes an arrangement ofcoils that generate locally a substantially radially-directed magneticfield, i.e., the field that would be produced by a single “magneticcharge” or magnetic pole. Here, we use the term “monopole” todistinguish this type of magnetic field from a dipole magnetic field(transversal or longitudinal). In some cases, the monopole antennaassembly generates quasi-stationary (relatively low frequency) magneticfields. In the example shown, the coils 37A and 37B, which are connectedin reverse polarity, are two parts of one monopole antenna assembly.Each coil by itself can be implemented as a standard longitudinalantenna. A monopole antenna can be implemented in another manner.

The polar plot in FIG. 3A shows an example of the antenna sensitivity,demonstrating unidirectional azimuthal selectivity. A combination of theresponses of each of the orthogonal transversal-dipole antennas with theresponse of the monopole antenna can give any of four possibledirections covering all quadrants of the transversal plane. Rotation ofthe drill string while drilling may cause an amplitude modulation of theazimuthally selective response and therefore an amplitude modulation ofthe NMR relaxation signal (e.g., a CPMG echo train). The amplitudemodulation parameters can indicate the azimuthal variations of the NMRproperties (e.g., the NMR porosity variations).

The coils 37A and 37B of the example monopole antenna shown in FIG. 3Bcan be used in combination with transversal-dipole antennas 35 and 36,for example, to achieve azimuthal selectivity. Either of the coils 37Aand 37B can also be used as a separate antenna (in addition to orwithout the transversal-dipole antennas 35, 36), for example, to gainSNR. In some cases, an NMR tool is implemented with a monopole antennaand a longitudinal magnet, without other antennas. For example, thetransversal-dipole antennas 35 and 36 may be omitted from the antennaassembly 31 in some cases.

FIG. 4A is a flowchart showing an example process 400 for obtaining NMRdata from a subterranean region; and FIG. 4B is a flowchart showinganother example process 420 for obtaining NMR data from a subterraneanregion. Each of the processes 400 and 420 can be performed independentof the other, or the processes 400 and 420 can be performed concurrentlyor in concert. For example, the processes 400 and 420 may be performedin series or in parallel, or one of the processes may be performedwithout performing the other.

The processes 400 and 420 can be performed by downhole NMR tools such asthe example NMR tools 200A, 200B, or 300B shown in FIGS. 2A, 2B and 3B,or by another type of NMR tool. The processes 400 and 420 can beperformed by a downhole NMR tool while the tool is disposed within awellbore during well system operations. For example, the downhole NMRtool can be suspended in the wellbore for wireline logging (e.g., asshown in FIG. 1B), or the downhole NMR tool can be coupled to a drillstring for NMR LWD (e.g., as shown in FIG. 1C).

Each of the processes 400 and 420 can include the operations shown inFIGS. 4A and 4B (respectively), or either of the processes can includeadditional or different operations. The operations can be performed inthe order shown in the respective figures or in another order. In somecases, one or more of the operations can be performed in series orparallel, during overlapping or non-overlapping time periods. In somecases, one or more of the operations can be iterated or repeated, forexample, for a specified number of iterations, for a specified timeduration, or until a terminating condition is reached.

At 402 in the example process 400 shown in FIG. 4A, the NMR tool ispositioned in a wellbore. In some cases, the NMR tool includes a magnetassembly to produce a magnetic field in a volume in the subterraneanregion about the wellbore. The volume can include, for example, all orpart of any of the volumes of investigation 17, 21, 24A, 24B, 34 shownin FIG. 2A, 2B or 3B, or another volume of interest. Generally, the NMRtool includes a magnet assembly to polarize nuclear spins in the volumeof interest, and an antenna assembly to excite the nuclear spins and toacquire an NMR signal based on the excitation.

At 404, polarization is generated in a volume about the wellbore. Thepolarization is generated by a static magnetic field, which is producedby the magnet assembly of the NMR tool in the wellbore. The polarizationrefers to the magnetic polarization of the nuclear spins in the volume.In other words, a portion of the nuclear spins becomes aligned with thestatic magnetic field, and the volume develops a bulk magnetic moment.In some cases, the static magnetic field is configured (e.g., by theshape and position of the magnet assembly) to produce longitudinalpolarization (e.g., parallel to the long axis of the wellbore) orpolarization having another orientation.

In some examples, the magnet assembly includes a central magnet (e.g.,the central magnet 12 shown in FIGS. 2A, 2B, 3B, or another type ofcentral magnet) and two end piece magnets (e.g., the end piece magnets11A, 11B shown in FIGS. 2A, 2B, 3B, or another type of end piecemagnet). In some cases, the magnets in the magnet assembly are permanentmagnets. As shown, for example, in FIG. 2A, the central magnet can be anelongate permanent magnet having a first axial end and a second,opposite axial end, with the first end piece magnet spaced apart fromthe first axial end of the central magnet, and with the second end piecemagnet spaced apart from the second axial end of the central magnet. Insome cases, the two end piece magnets have a common magnetic fieldorientation, and the central magnet has the opposite magnetic fieldorientation (e.g., such that both end piece magnets have a magneticfield orientation that is orthogonal to the magnetic field orientationof the central magnet).

At 406, circular-polarized excitation is generated in the volume aboutthe wellbore. The circular-polarized excitation is produced in thevolume by an antenna assembly. For example, the antenna assembly can beenergized by a radio-frequency current, which produces a radio-frequency(RF) magnetic field in the volume about the wellbore. The RF magneticfield generated by the antenna assembly manipulates the nuclear spins toproduce an excited spin state that has circular polarization. In otherwords, the resulting spin polarization has a circular (orcircumferential) orientation in the volume about the wellbore.

In some examples, the antenna assembly includes orthogonaltransversal-dipole antennas. The antenna assembly 13 shown in FIGS. 2Aand 2B and the antenna assembly 31 shown in FIG. 3B are examples ofantenna assemblies that include two orthogonal transversal-dipoleantennas. Each antenna 15, 16 in the example antenna assembly 13 canindependently produce a transversal-dipole magnetic field, for example,by conducting radio-frequency current. In the examples shown, eachtransversal-dipole magnetic field has a transverse orientation withrespect to the longitudinal axis of the NMR tool. In other words, thetransversal-dipole magnetic field is oriented orthogonal to the longaxis of the wellbore.

In the example shown, the transversal-dipole magnetic field produced bythe antenna 15 is orthogonal to the transversal-dipole magnetic fieldproduced by the other antenna 16. For example, in a Cartesian coordinatesystem of three mutually-orthogonal directions, the longitudinal axis ofthe NMR tool can be considered the “z” direction, and thetransversal-dipole magnetic fields (produced by the antennas 15, 16) areoriented along the “x” and “y” directions, respectively.

In some implementations, other types of excitation are produced by theNMR tool. For example, in some cases, the circular-polarized excitationis produced in a first sub-volume (e.g., the volume of investigation 21in FIG. 2B) by the orthogonal transversal-dipole antennas, andexcitation having another orientation is produced in second and thirdsub-volumes (e.g., the volumes of investigation 24A, 24B in FIG. 2B)that are spaced apart from the axial ends of the first sub-volume. Theexcitation in the second and third sub-volumes can be produced, forexample, by a longitudinal-dipole RF field generated by other antennaassemblies (e.g., by antennas 23A and 23B in FIG. 2B). The distinctsub-volumes may be useful for different purposes. For example, the firstsub-volume can be elongate (parallel to the long axis of the wellbore),to acquire NMR data from the first sub-volume while the NMR tool movesalong the wellbore (e.g., while tripping a drill string). In some cases,the other sub-volumes can be positioned to acquire NMR data for mudfiltrate invasion profiling or other applications.

At 408, an NMR signal is acquired by quadrature coil detection. The NMRsignal is based on the excitation generated at 406. The NMR signal canbe, for example, an echo train, a free induction decay (FID), or anothertype of NMR signal. In some cases, the acquired NMR data includes T1relaxation data, T2 relaxation data, or other data. The NMR signal canbe acquired by the antenna assembly that produced the excitation or byanother antenna assembly. In some cases, an NMR signal can be acquiredin multiple sub-volumes.

Quadrature coil detection can be performed by the orthogonaltransversal-dipole antennas. Quadrature coil detection can be performedby using two orthogonal coils, each picking up the signal induced bycircular polarized nuclear magnetization (the signal in the coils have90 degree phase difference). Even if during transmission only one coilis used (e.g., producing linear polarized RF magnetic field), thenuclear magnetization can still be circular polarized. Quadrature coiltransmission (two orthogonal coils driven by RF currents having 90degree phase difference) can enable circular polarized excitation, whichcan help to reduce power consumption compared to a linear polarizedexcitation in some cases. Quadrature coil detection can be used, forexample, to increase signal-to-noise ratio (SNR) when exciting only onecoil (not using circular polarized excitation to simplify hardware), orcircular polarization can be used to save power while detecting signalswith one coil. In some cases, both circular polarization and quadraturecoil detection can be used to save power and increase SNR. In somecases, the use of circular polarization or quadrature coil detection (orboth) is efficient when the mutually orthogonal antennas aresubstantially identical. This is possible in the example magnet/antennaconfiguration that has a longitudinal dipole magnet and two transversalantennae. Other configurations that have one of the two antennae lessefficient than the other, although allowing for mutually orthogonalantennae, may not provide the same advantages in some cases.

At 410, the NMR data are processed. The NMR data can be processed toidentify physical properties of the subterranean region or to extractother types of information. For example, the NMR data may be processedto identify density, viscosity, porosity, material content, or otherproperties of the subterranean region about the wellbore.

At 422 in the example process 420 shown in FIG. 4B, the NMR tool ispositioned in a wellbore, and at 424 polarization is generated in avolume about the wellbore. Operations 422 and 424 in FIG. 4B are similarto operations 402 and 404 shown in FIG. 4A. For example, the NMR toolincludes a magnet assembly to polarize nuclear spins in the volume ofinterest, and an antenna assembly to excite the nuclear spins and toacquire an NMR signal based on the excitation. The polarization can beproduced at 424 in the manner described with respect to operation 404 ofFIG. 4A and by the same type of magnet assembly; or polarization can beproduced at 424 in another manner or by another type of magnet assembly.

At 426, excitation is generated in a volume about the wellbore. Theexcitation is produced in the volume by an antenna assembly. Forexample, the antenna assembly can be energized by a radio-frequencycurrent, which produces a radio-frequency (RF) magnetic field in thevolume about the wellbore. The RF magnetic field generated by theantenna assembly manipulates the nuclear spins to produce an excitedspin state. In some instances, the spin state has a higher excitation ina selected azimuthal direction, such that the level of spin excitationvaries along a circular (or circumferential) direction about thewellbore, for example, due to an azimuthally-selective RF magneticfield.

In some examples, the antenna assembly includes a transversal-dipole andmonopole antenna assembly. The antenna assembly 31 shown in FIG. 3B isan example of an antenna assembly that includes a transversal-dipole andmonopole antenna assembly. In the example shown in FIG. 3B, thetransversal-dipole and monopole antenna assembly includes two orthogonaltransversal-dipole antennas 35 and 36 in a central region, and amonopole antenna that includes a first coil 37A at a first axial end ofthe transversal-dipole antennas 35 and 36 and a second coil 37B at asecond, opposite axial end of the transversal-dipole antennas 35 and 36;the coils 37A and 37B of the monopole antenna are arranged with oppositepolarity.

At 428, an azimuthally-selective NMR signal is acquired. The NMR signalis based on the excitation generated at 426. The NMR signal can be, forexample, an echo train, a free induction decay (FID), or another type ofNMR signal. In some cases, the acquired NMR data includes T1 relaxationdata, T2 relaxation data, or other data. The NMR signal can be acquiredby the antenna assembly that produced the excitation or by anotherantenna assembly. In some cases, the NMR signal is acquired by anantenna assembly having azimuthally-selective sensitivity, such as, atransversal-dipole and monopole antenna assembly.

In some implementations, the azimuthally-selective NMR signal isacquired as a combination of multiple NMR signal acquisitions. Thesignal acquisitions can include, for example, acquisitions by one ormore transversal-dipole antennas and one or more monopole antennas. Thesignals can be combined to enable azimuthally-resolved measurements ofthe volume about the wellbore. For example, in some cases, a propercombination of the responses of each of the orthogonaltransversal-dipole antennas with the response of the monopole antennacan give any of four possible directions covering all quadrants of thetransversal plane.

At 430, the NMR data are processed. The NMR data can be processed toidentify physical properties of the subterranean region or to extractother types of information. For example, the NMR data may be processedto identify density, viscosity, porosity, material content, or otherproperties of the subterranean region about the wellbore. In some cases,the NMR data are processed to identify azimuthal variations in thesubterranean region about the wellbore. For example, rotating the NMRtool may cause an amplitude modulation of the azimuthally-selectiveresponse. The amplitude modulation parameters can indicate the azimuthalvariations of the properties affecting the NMR signal (e.g., porosity,density, viscosity, material content, etc.).

While this specification contains many details, these should not beconstrued as limitations on the scope of what may be claimed, but ratheras descriptions of features specific to particular examples. Certainfeatures that are described in this specification in the context ofseparate implementations can also be combined. Conversely, variousfeatures that are described in the context of a single implementationcan also be implemented in multiple embodiments separately or in anysuitable subcombination.

A number of examples have been described. Nevertheless, it will beunderstood that various modifications can be made. Accordingly, otherimplementations are within the scope of the following claims.

1. A nuclear magnetic resonance (NMR) tool for use in a wellbore in asubterranean region, the NMR tool comprising: a magnet assembly toproduce a magnetic field in a volume in a subterranean region; and anantenna assembly to produce an excitation in the volume, and to acquirean azimuthally-selective response from the volume based on theexcitation, the antenna assembly comprising a transversal-dipole antennaand a monopole antenna.
 2. The NMR tool of claim 1, wherein the monopoleantenna comprises: a first coil at a first axial end of thetransversal-dipole antenna; and a second coil at a second, oppositeaxial end of the transversal-dipole antenna, and the first and secondcoils are oriented along a common axis and have opposite polarity. 3.The NMR tool of claim 1, wherein the excitation is produced by thetransversal-dipole antenna and the monopole antenna.
 4. The NMR tool ofclaim 1, wherein the magnet assembly comprises: a central magnet havinga first axial end and a second, opposite axial end; a first end piecemagnet spaced apart from the first axial end of the central magnet; anda second end piece magnet spaced apart from the second axial end of thecentral magnet.
 5. The NMR tool of claim 4, wherein the magnet assemblycomprises a permanent magnet assembly, and the central magnet and thefirst and second end piece magnets each comprise one or more permanentmagnets.
 6. The NMR tool of claim 4, wherein the central magnet definesa first magnetic field orientation, and the first and second end piecemagnets each define a second magnetic field orientation that isorthogonal to the first magnetic field orientation.
 7. The NMR tool ofclaim 1, wherein the magnet assembly and the antennae are configured tooperate within a wellbore in the subterranean region during drillingoperations.
 8. The NMR tool of claim 1, wherein: the volume comprisesmultiple distinct sub-volumes, the multiple distinct sub-volumescomprise a first sub-volume that is elongate in a first directionparallel to a longitudinal axis of the NMR tool, the magnetic field inthe first sub-volume being substantially uniformly oriented in the firstdirection; and the NMR tool comprises multiple antenna assemblies atrespective locations along the longitudinal axis, each antenna assemblyto detect an NMR response from a respective one of the distinctsub-volumes.
 9. The NMR tool of claim 1, wherein the magnet assembly andantennae assembly are operable to acquire an NMR signal while drilling.10. A method of obtaining nuclear magnetic resonance (NMR) data from asubterranean region, the method comprising: producing a magnetic fieldin a volume in a subterranean region by a magnet assembly in a wellbore;and producing an excitation in the volume; and acquiring anazimuthally-selective response from the volume based on the excitation,the response acquired by an antenna assembly comprising atransversal-dipole antenna and a monopole antenna.
 11. The method ofclaim 10, further comprising identifying azimuthal variations in thevolume based on the response.
 12. The method of claim 10, wherein: theantenna assembly comprises transversal-dipole antennas; the monopoleantenna comprises a first coil at a first axial end of thetransversal-dipole antennas, and a second coil at a second, oppositeaxial end of the transversal-dipole antennas; and the first and secondcoils are oriented along a common axis and have opposite polarity. 13.The method of claim 12, wherein the excitation is produced by at leastone of the monopole antenna or one or more of the transversal-dipoleantennas.
 14. The method of claim 10, wherein a downhole NMR toolcomprises the magnet assembly and the antenna assembly, and theexcitation is produced and the response is acquired while the downholeNMR tool is disposed in a wellbore in the subterranean region.
 15. Themethod of claim 14, wherein the NMR tool is couple to a drill string andoperates during drilling operations in the wellbore.
 16. A drill stringassembly comprising a downhole Nuclear Magnetic Resonance (NMR) tooldisposed in a wellbore in a subterranean region, the downhole NMR toolcomprising: a magnet assembly to produce a magnetic field in a volume ina subterranean region; and a transversal-dipole and monopole antennaassembly to obtain an NMR response from the volume.
 17. The drill stringassembly of claim 16, wherein the transversal-dipole and monopoleantenna assembly is operable to obtain a unidirectionalazimuthally-selective NMR response from the volume.
 18. The drill stringassembly of claim 16, wherein the downhole NMR tool comprises multipletransversal-dipole orthogonal antennas to: produce circular polarizedexcitation in a first sub-volume; and acquire a response from the firstsub-volume by quadrature coil detection.
 19. The drill string assemblyof claim 16, wherein: the volume comprises multiple distinctsub-volumes, the multiple distinct sub-volumes comprises a firstsub-volume that is elongate in a first direction parallel to alongitudinal axis of the downhole NMR tool, and the magnetic field inthe first sub-volume is substantially uniformly oriented in the firstdirection; and the downhole NMR tool comprises multiple antennaassemblies at respective locations along the longitudinal axis, eachantenna assembly operable to detect an NMR response from a respectiveone of the distinct sub-volumes.
 20. A nuclear magnetic resonance (NMR)tool for use in a wellbore in a subterranean region, the NMR toolcomprising: a magnet assembly to produce a magnetic field in a volume ina subterranean region, the magnetic field in the volume orientedparallel to a longitudinal axis of the NMR tool; and an antenna assemblyto produce an excitation in the volume and to acquire a response fromthe volume based on the excitation, the antenna assembly comprising amonopole antenna.